Exploration Of Highly Efficient Catalysts For Hydrogenation Of Nitroarenes To Amines And Oxidative Coupling Of Alcohols And Amines To Imines

Aromatic amines are important intermediates for the synthesis of various medicines,pesticides,dyes,polymers and fine chemicals,having wide applications.Aromatic amines are mainly obtained by selective hydrogenation of nitroarenes.Traditional hydrogenation methods include metal reduction,alkali sulfide reduction,electrochemical reduction,CO/H₂O reduction and catalytic hydrogenation.The catalytic hydrogenation is environmentally friendly,because it does not need to use the strong acid and alkali media.So this method has become the first choice for the hydrogenation of nitroarenes to prepare aromatic amines.Supported Pt-based catalysts exhibit high catalytic activity in this reaction,but when there are other reducible groups in the reaction substrate such as C=C,C=O,-X and so on,these groups might possibly be reduced during the process of nitro hydrogenation.In order to overcome this challenge,low-loading Pt/Fe₂O₃ catalysts were prepared by colloidal deposition method.The catalytic performance and the reaction mechanism of the catalysts for selective hydrogenation of nitroarenes was investigated.The results showed that the catalytic activity of 0.2 wt%Pt/Fe₂O₃ catalyst in selective hydrogenation of nitroarenes was higher than that of commercial 5 wt%Pt/C.After recycling test,the size and dispersion state of Pt nanoparticles and structure of Fe₂O₃support remained unchanged,showing that the catalyst exhibited high stability.The characterization results show that not only the high efficient hydrogen dissociation ability of Pt nanoparticles but also the oxygen vacancies and the appropriate redox property of Fe₂O₃ support are the key factors for the selective hydrogenation of nitroarenes under mild conditions.Further,as an important nitrogen-containing compound,imine has an unsaturated double bond of C=N,which is widely used in the fields of medicine,fine chemical industry and agriculture.Coupling oxidation of alcohols and amines to imine is environmentally friendly,because the by-product of this process is only water.Supported magnesium oxide catalysts were prepared by using nitric acid treated mesoporous carbon as support and magnesium oxide as active component.The catalyst with excellent catalytic performance was obtained by discussing the dispersibility and loading amount of MgO.The results showed that the catalytic performance of supported magnesium oxide for the coupling oxidation of alcohols and amines to imine was significantly higher than that of bulk magnesium oxide.Compared with the FeO_x/MC and CeO_x/MC catalysts reported earlier by our group,after two hours of reaction,the activity of 5 wt%MgO_x/MC is higher than that of 5 wt%FeO_x/MC,which is similar to that of 10 wt%CeO_x/MC,while MgO_x/MC has both advantages of cost and environmental friendliness.The characterization results showed that there are a lot of oxygen defects on the surface of MgO_x/MC catalyst,which is beneficial to the activation of molecular oxygen and the increase the overall reaction rate.In conclusion,nano-catalysts with excellent catalytic performance were prepared for use of synthesis and utilization of amines.These explorations will provide reference for further development of catalysts for hydrogenation of nitroarenes to amines and coupling oxidation of alcohols and amines to imine.